In FM-BIEM, we follow the techniques proposed by Song et
al.[76] to truncate the infinite series taking P terms, where

(3.132)

k is the wave number and R is the diameter of a sphere which
circumscribes a cell. One has
,
where D is the edge
length of a cell. This means that we must change P as the level
changes. Also, P increases as the size of a cell or the wave number
becomes large. This property leads to a lot of computational cost when
one deals with high frequency problems or large-scale problems. In order to
overcome this problem some techniques are proposed by several authors
(e.g. Rokhlin et al.[71] or Greengard et al.[34]).
However, we do not use these techniques in this thesis since we are
interested in low frequency problems.